How to use the vega-projection.getProjectionPath function in vega-projection

prototype.transform = function(_, pulse) {
  var out = pulse.fork(pulse.ALL),
      shape = this.value,
      as = _.as || 'shape',
      flag = out.ADD;

  if (!shape || _.modified()) {
    // parameters updated, reset and reflow
    this.value = shape = shapeGenerator(
      getProjectionPath(_.projection),
      _.field || field('datum'),
      _.pointRadius
    );
    out.materialize().reflow();
    flag = out.SOURCE;
  }

  out.visit(flag, function(t) { t[as] = shape; });

  return out.modifies(as);
};

prototype.transform = function(_, pulse) {
  var out = pulse.fork(pulse.ALL),
      path = this.value,
      field = _.field || identity,
      as = _.as || 'path',
      flag = out.SOURCE;

  function set(t) { t[as] = path(field(t)); }

  if (!path || _.modified()) {
    // parameters updated, reset and reflow
    this.value = path = getProjectionPath(_.projection);
    out.materialize().reflow();
  } else {
    flag = field === identity || pulse.modified(field.fields)
      ? out.ADD_MOD
      : out.ADD;
  }

  var prev = initPath(path, _.pointRadius);
  out.visit(flag, set);
  path.pointRadius(prev);

  return out.modifies(as);
};